How many Gigs?

Solution
Depends on how much you want to spend. 8GB is enough for most stuff, 16GB is nice and smooth though. Considering you are getting a new fancy DDR4 CPU, I would go 16GB off the start in 2x8GB chips.

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+1 If budget permits would go 16 as that will prob be the entry level in less than year, and will prob face upgrading (even for gaming withing that year)